Process for regenerating silver salt solutions



United States Patent The present invention relates to a process for regenerating silver salt solutions.

It is known that acetylene'hydrocarbons, especially acetylene react with silver ions to form very explosive silver acetylides. In German Patent, 1,027,658, disclosing isolation of olefins from olefin-containing gases by.

absorption in silver salt solutions, it is recommended to remove traces of acetylenes included in these gases by previous partial hydrogenation in order to avoid accumulation of relatively great amounts of acetylides in the silver salt solution. This must be done as silver acetylides partially dissolve in concentrated silver salt solutions before they are being precipitated as solid silver acetylides. In other words, the presence of silver acetylides in those solutions does not immediately manifest itself in the formation of a precipitate.

It is knownthat silver salt solutions are used for selectively absorbing olefins from gas mixtures. The gas mixtures subjected to such absorption always contain impurities which enrich in the absorption solution and imply its regeneration. These impurities which may be either soluble or insoluble in the silver salt solutions include the acetylene hydrocarbons, more especially acetylene, its dimers and the mono-alkyl derivatives of these hydrocarbons, which react with the silver salt solution to form silver acetylides, and other unsaturated hydrocarbons, primarily poly-unsaturated hydrocarbons, that form polymers in the solution.

We have now found that silver acetylides and resinous substances formed from polymerized olefinic hydrocarbons or their derivatives can be removed from aqueous silver salt solutions and thelatter regenerated by subjecting these solutions to thermal treatment at a temperature of at least 50 C. 1

During that treatment, metallic silver separates from the solution under mild conditions while silver acetylides included in the solution disappear simultaneously as can readily be proved by physical or chemical means. At the same time, non-volatile organic substances, if any, which may have been formed by side-reactions, such as foamforming substances or polymers, are removed together with the silver acetylides with separation of silver. The thermal treatment according: to this invention removes from the solution, for example silver acetylides, silver methylacetylide, silver vinylacetylide, silver diacetylide and silver. salts of mono-lower alkyl acetylenes, such as ethyl acetylene, propyl acetylene or butyl acetylene, resins and polymers which may have been formed from olefinic hydrocarbons such as butadiene, isoprene, chloroprene or allene and have accumulated or precipitated in said solution. I

The silver separated can readily be redissolved using an oxidizing agent, preferably hydrogen peroxide.

In order to avoid that during the thermal treatment -more silver than corresponds to the decomposition of the silver acetylides is separated, the silver salt solution to be regenerated is freed before or during thermal treatment from volatile organic ingredients by stripping with steam or a gas inert towards the silver salt solution, such. as nitrogen, noble gases or carbonvdioxide. Stripping is advantageously carried out at a raised temperature,

preferably at the boiling temperature of the silver salt solution and, if desired, under reduced pressure.

As mentioned above, the silver salt solution is subjected to the heat treatment according to this invention at a temperaure above 50 C., especially above C. and more especially between and 200 C. and, if desired under pressure, preferably that pressure which corresponds to the vapor pressure of the solution at its boiling point.

The heat treatment should be carried out until the desired low proportion of acetylides in the solution has been obtained. The time of treatment depends on the temperature used and the initial respectively desired concentration of the acetylides in the solution. The time of treatment is the shorter the higher the temperature, the lower the initial coneentrationg and the higher the admissible final concentration of silver acetylides in the silver salt solution. In view of. all these factors, the treatment may be carried out for a period of between 1 minute up to 10 hours. The conditions advantageously should be so selected that the heat treatment is complete within less than 1 hour. Under conditions desirable from a technological point of view, for example at relatively high temperatures and with not too high a concentration of acetylides in the solution, the treatment will generally be completed within a period of some minutes up to M; hour..

In order to accelerate the decomposition of silver acetylides, the thermal treatment may be carried out in the.

presence of an oxidizing agent, for example air, oxygen, ozone, hydrogen peroxide, oxidizing nitrogen-oxygen acids or oxidizing nitrogen oxides, such as NO, N0 N 0 N 0 N 0 dilute, concentrated or fuming nitric acid, nitrates or nitrites among whichthe water-soluble or more especiallyythe alkali metal or alkaline earth metal nitrates or nitrites or the corresponding silver salts are preferred.

The thermal treatment may also be carried out by injecting the silver salt solution to be regenerated into a superheated gas current, advantageously a current of one of the above mentioned inert gases or steam," at atmospheric pressure or super-pressure and thus decomposing the impurities included in the solution; The current of gas should have a temperature of at least 100 C., preferably 150-250 C.

The process of this invention offers the advantage that a silver salt solution used for continuous ethylene absorption (cf. German Patent 1,027,658) moved (partial current) intermittently or continuously to decompose in the partial current branched, off the acetylides and resinous impurities so that the content of dissolved silver acetylides in the bulk of the solution does not exceed a certain degree of saturation, preferably about 40% The time required for regeneration may be shortened and the regeneration temperature reduced to about 50 to 150 C. by adding to the silver salt solution before or during the heat treatment a solid oxidizing agent, such as alkali metal or silver permanganate or silver oxide, or by producing these agents intermediately in the solution during the heat treatment. When these solid oxidizing agents are used, it isadvantageous to Work at about 100 C., i.e., the boiling point of the solution under atmospheric pressure.

starting material which is transformed by means of molecular oxygen, eg in the form of air, pure oxygen'or ozone into its oxide and which transmits its oxygen to the organic impurities.

may in part be re-.

dr'ogenperoxide.

Whensilver oxide is used, care must of course bev taken to ensure that the silver salt solution oontains less free acid than is' neccssary to dissolvethesilver oxide.

The silver'salt solutions which eanbe regenerated by the process of this invent-ion ineludc, for example, aque-v ous solutions of silver, borol'luoride, silver fluorosilicatc,

' silver nitrate, silver perchlorate, 'silver fluoride, silver trifluoroacetate, silver antimony hexafluoride, silver arsenichexafldoride-or mixture oi: these salts.

I The following examples serve to illustrate 'the:inven-' tion, lbut they are not intended-tolimit it thereto, the parts 1 k and percentages are.

by weight unless otherwise stated: EXanzple I 1'0-cc.'.ot.a silver borofiu'oridesolution containing 6g: dissolved silver were contacted-with 50cc. acetylene which dissolved-without a precipitatebeingforrned. l Afterheat ing for '1 hour to-200 'C.'in ;:an autoclave, silver had separated. which wastdissolved' by adding the.-. calculated-- amountHgO-. The silver salt solutionjso treatedwas substantially'freefrom dissolved silveracetylidest Example 2 the solution had dropped to less thani5% OPUS-initial volume 100 parts by volume acctylene;=wasflpassed c'ounterj currently through a silver :boro-fluoridesolution (ct. Example 5), the ethylene and; acetylene being selectively value; Example 7 A, gas composed of about 80% by'volume'carbon' monoxide, methane and hydrogen anda'bo'ut by volume ethylene andrcontaining'in-1,000,000 parts by absorbed. After ethylene removal in vacuo, by heating A- current of gas-'(composed of -20r-p arts. volume? ethylene and 80' parts by volume ethane and containing 0.002% by volume acetylene) was washed at atmospheric pressurexwitha saturatedsilver nitrate-solution and the,-

ethylenezwas liberated 'from' the silver nitratesolut-ionq I by-boiling under,reduced' pressure (20 mm.-.mercury).

A partial currentofthe silver nitratesolutionwas treate ed with steam-and thus -freed"fromtraces 'ofidissolvedethylene. Subsequently,' th'e whole was heated at 2009' C. while." adding small amounts of. nitrieacidzand mainl tainedlat that. temperature until-.silver acetylide could nolonger be; detecteil Theihot solution wastreleased to atmospheric pressure" whereby oxidation" productsv and.

nitrogen-oxides were removedland. the solution was-re turnedrito the wash process. The contentofi acetylene in the silver-salt solution was thus maintainedat less than- Ice. gaseous acetylene per cc.'solution.

Example 3" I A gas composed of 30% 'by volume ethylene, 20% by volume-propylene, 1% 'bylvolume acetylene, rest nitrogenwas passed through a solution of silver borofluoride incon centrated-hydro-fluoboric acid, containing635v g.- silver or-stripping, the acetylide-containing'i silver salt solution was heated for 2 hours ,at 90 C, with an excessof silver "oxide 'until-it-he acetylide was completely destroyed by oxidatiom The silver oxide in excess and the silver formed" were then removed by suction-filtration. ,T he filter residue was then dissolved rwith semi-concentrated nitric acid and the silver oxide was again precipitated by meansof a sodium hydroxide solution; Thesilver. oxide was washed and could the'nwagain be used for oxidizing acetylides.

' Example 8 7'liters AgiiF solution" were gas-treated under 5 atmospheres with 2lm. /h.of Fa'C -hydrocarbon fraction butylene, 42.5%butadiiene, remainder C -paraf Y fines). 1 The pressure was. reduced "and the'nnsaturated hydrocarbons were. thus liberated. After 24-hours of operation, thesilver, salt solution contained 4.6% C.

The desor'bedga seswere analyzed and theloss ofz'butadieneuwasdeterminedwith 15%: Heat'treatment .for 2 hoursof the carbon-containing solution at 200C. reduced 'C-contentto 0.3'to 0.15%. v 1 o I 'We-claim: v

1. 'In a process of selectively absorbing olefins from a gaseous mixture containing an olefin and an aeetylenic impurity byan aqueous solution of a water-soluble'silver salt of silver-and an anion selected from the'group consisting of borofiuoride; fluorosilic-ate, nitrate, perchlorate,

- fluoride, tritluoroacetate,v antimony vhexafluoride, arsenic per liter. After" 8 cc acetylene had been. absorbed per fcc. si-lver'salt solution, the solution was .heated at 120 C. and boiled. After heating .for' Ihour at-thattempera- 'ture, dissolved silver acetylide was no longer detectable.

' Y The silyerwhich had precipitated in a minor proportion could. be-dis'solved .by add-ing'the calculatedainount of hy- "Example .4

Aysilver borofiuoride" solution containing per liter 600 g. dissolved silver, was used. 4 2 liters of that solution containing jacetylide andfsoluble organic substances in a proportionwhich corresponded to'a content" ofv 6' liters acetylenc'and hornologs thereof *were admixed at C.

with .antaqueous solution'aof' 192 'g'; silver permanganate;

- taminants-areeffectivelyconverted to oxidation and reduc Thewhole'was thoroughly stirred and separated pyrolusite was filtered :otfl

appeared" I f Example 5 A silver .borofluorid'e. solution containing pe-r liter 750' g. *dissolved silver. (pH valueof'the solution: 4) was used for absorbing acetylene. 2.1mm: suchsolution whieh had'been neutralized with silver oxide-and. contained '4 liters acetylenic hydrocarbons in the. form" of bound i acetylides," were mixed-with 200 ccrof' a silver catalyst. containing..-10%.silver on' silic'a'gel; The wholewasthen gas-treated. through: a'frit with 24to 3 g. ozone/hour.

After .5 hours, the-acetylene content of the solution-had" dropped to less thanl0% Of the initial value.

Thev acetylide had then substantially dishexafluoride and mixtures thereof, said solution being .contaminated with a dissolved silver acetylide selected fromthegroup' consisting ofQsilve-r acetylide, silverlower alkyl acetylide, silver vinylacetylide;and-silverdiacetylide formed-by silver ions. from s'aid solution and an acetylenic impurity absorbed in said solution fromi saidgaseous mix-.

tureand contaminated with and unsaturated polymer formed from polymerized-olefinic hydrocarbons of 3 to 5 carbon atoms absorbed insaid-solntion from Saidgasebus-mixture, the steps of regenerating said .aqueouysolution which'co'mprise heating said solution to a temperaturea bove 100]C, to ox-idize'the acetylideof said silver acetylide, to oxidize said polymer and to reduce the silver of said silveracetylide to'freeisilver whereby said contion'products', and oxidizing saidfree silver to silver ions to regeneratcsaid solution.

2." The proeessxof 'claim,1,., wherein' the solution is I atomizedintoasuper-heated gas stream. v 3. A processias defined in claim 1 whereinvolatile oxidation products .of'said silver acetylide and said polymer formed during regeneration of the solution are stripped therefrom" by passing. through the solution a gas 7 inert-thereto.

4.-1In a process of: selectively absorbing olefins from a gaseous mixture containing an*olefin" and an acetylenic impurity by an aqueous solution of'a water-soluble, silver salt of silver and an anion selected from the group consisting. of borofluoride, fluorosilicate;nitrate, perchlorate, fluoride, trifiuoroacetate, antimony hexafluoride, arsenic hexafliioride andmixtures thereof,said solution being con-I taminated with a dis'solved silver. acetylide selected from the group consisting of silver acetylide, silver lower alkyl acetylide, silver vinylacetylide and silver diacetylide formed by silver ions from said solution and an acetylenic impurity absorbed in said solution from said gaseous mixture and contaminated with an unsaturated polymer formed from polymerized olefinic hydrocarbons of 3 to 5 carbon atoms absorbed in said solution from said gaseous mixture, the stepsof regenerating said aqueous solution which} comprise heating said .solution to a temperature above 50 C. in the presence of an oxidizing agent sewhereby said contaminants are etfectively converted to oxidation and reduction products, and oxidizing said free silver to silver ions to regenerate said solution.

. 5. A process for regenerating an aqueous solution of a water-soluble silver salt of silver and an anion selected from the group consisting of born-fluoride, fiuorosilicate,

nitrate, perchlorate, fluoride, trifiuoroacetate, antimony hexafluoride, arsenic hexafiuoride and mixtures thereof, which solution is contaminated with a dissolved silver acetylide selected from the group consisting of silver acetylide, silver-lower alkyl acetylide, silver vinylacctylide and silver diacety-lide formed by silver ions from said solution and an acetylenic impurity absorbed in said solution and contaminated with an unsaturated polymer formed from polymerized olefinic hydrocarbons of 3 to 5 carbon atoms absorbed in said solution, which process comprises heating said solution to a temperature above 100 C. to oxidize the acetylide of said silver acetylide, to oxidize said polymer and to reduce the silver of said silver acetylide to free silver whereby said contaminants are effectively converted to oxidation and reduction products,,and oxidizing said free silver to silver ions to regenerate said solution.

6. A process for regenerating an aqueous solution of a water-soluble silver salt of silver and an anion selected from the group consisting of borofiuoride, fluorosilicate,

nitrate, perchlorate, fluoride, trifiuoroacetate, antimony hexafluoride arsenic hexafluoride and mixtures thereof, which solution is contaminated with a dissolved silver acetylide. selected from the group consisting of silver acetylide, silver lower alkyl acetylide, silver vinylacetylide and silver diacetylide formed by silver ions from said solution and an acetylenic impurity absorbed in said solution and contaminated with an unsaturated polymer formed from polymerized olefinic hydrocarbons of 3'to 5 carbon atoms absorbed in said solution, which process comprises heating said solution to a temperature above C. in the presence of an oxidizing agent selected from the group consisting of air, oxygen, ozone,-hydrogen peroxide, a nitrogen oxide, a nitrogen-oxygen acid, an

alkali metal permanganate, silver permanganate, silver oxide and mixtures thereof to oxidize the acetylide of said silver acetylide, to oxidize said polymer and to reduce the silver of said silver acetylide to free silver whereby said contaminants are effectively converted to oxidation'and reduction products, and oxidizing said free silver to silver. ions to regenerate said solution.

7. A process according to claim 1 which further comprises continuously withdrawing a portion of said contaminated solution, regenerating said withdrawn portion 'by the steps defined in claim 1, and recycling to the solution the portion so regenerated.

8. A process according to claim 5 which further comprises continuously withdrawing v a portion of said contaminated solution, regenerating said withdrawn portion by the steps defined in claim 5, and recycling to the solution the portion so regenerated.

References Cited by the Examiner UNITED STATES PATENTS German application, 1,027,658, printed April 10, 1958. a

MAURiC-E A. BRINDISI, Primary Examiner. 

1. IN A PROCESS OF SELECTIVELY ABSORBING OLEFINS FROM A GASEOUS MIXTURE CONTAINING AN OLEFIN AND AN ACETYLENIC IMPURITY BY AN AQUEOUS SOLUTION OF A WATER-SOLUBLE SILVER SALT OF SILVER AND AN ANION SELECTED FROM THE GROUP CONSISTING OF BOROFLUORIDE, FLOROSILICATE, NITRATE, PERCHLORATE, FLUORIDE, TRIFLUOROACETATE, ANTIMONY HEXAFLUORIDE, ARSENIC HEXAFLUORIDE AND MIXTURES THEREOF, SAID SOLUTION BEING CONTAMINATED WITH A DISSOLVED SILVER ACETYLIDE SELECTED FROM THE GROUP CONSISTING OF SILVER ACETYLIDE, SILVER LOWER ALKYL ACETYLIDE, SILVER VINYLACETYLIDE AND SILVER DIACETYLIDE FORMED BY SILVER IONS FROM SAID SOLUTION AND AN ACETYLENIC IMPURITY ABSORBED IN SAID SOLUTION FROM SAID GASEOUS MIXTURE AND CONTAMINATED WITH AN UNSATURATED POLYMER FORMED FROM POLYMERIZED OLEFINIC HYDROCARBONS OF 3 TO 5 CARBON ATOMS ABSORBED IN SAID SOLUTION FROM SAID GASEOUS MIXTURE, THE STEPS OF REGENERATING SAID AQUEOUS SOLUTION WHICH COMPRISE HEATING SAID SOLUTITON TO A TEMPERATURE ABOVE 100*C. TO OXIDIZE THE ACETYLIDE OF SAID SILVER ACETYLIDE, TO OXIDIZE SAID POLYMER AND TO REDUCE THE SILVER OF SAID SILVER ACETYLIDE TO FREE SILVERE WHEREBY SAID CONTAMINANTS ARE EFFECTIVELY CONVERTED TO OXIDATION AND REDUCTION PRODUCTS, AND OXIDIZING SAID FREE SILVER TO SILVER IONS TO REGENERATE SAID SOLUTION.
 4. IN A PROCESS OF SELECTIVELY ABSORBING OLEFINS FROM A GASEOUS MIXTURE CONTAINING AN OLEFIN AND AN ACETYLENIC IMPURITY BY AN AQUEOUS SOLUTION OF A WATER-SOLUBLE SILVER SALT OF SILVER AND AN ANION SELECTED FROM THE GROUP CONSISTING OF BOROFLUORIDE, FLUOROSILICATE, NITRATE, PERCHLORATE, FLUORIDE, TRIFLUOROACETATE, ANTIMONY HEXAFLUORIDE, ARSENIC HEXAFLUORIDE AND MIXTURES THEREOF, SAID SOLUTION BEING CONTAMINATED WITH A DISSOLVED SILVER ACETYLIDE SELECTED FROM THE GROUP CONSISTING OF SILVER ACETYLIDE, SILVER LOWER ALKYL ACETYLIDE, SILVER VINYLACETYLIDE AND SILVER DIACETYLIDE FORMED BY SILVER IONS FROM SAID SOLUTION AND AN ACETYLENIC IMPURITY ABSORBED IN SAID SOLUTION FROM SAID GASEOUS MIXTURE AND CONTAMINATED WITH AN UNSATURATED POLYMER FORMED FROM POLYMERIZED OLEFINIC HYDROCARBONS OF 3 TO 5 CARBON ATOMS ABSORBED IN SAID SOLUTION FROM SAID GASEOUS MIXTURE, THE STEPS OF REGENERATING SAID AQUEOUS SOLUTION WHICH COMPRISE HEATING SAID SOLUTION TO A TEMPERATURE ABOVE 50*C. IN THE PRESENCE OF AN OXIDIZING AGENT SELECTED FROM THE GROUP CONSISTING OF AIR, OXYGEN, OZONE, HYDROGEN PEROXIDE, A NITROGEN OXIDE, A NITROGEN-OXYGEN ACID, AN ALKALI METAL PERMANGANATE, SILVER PERMANGANATE, SILVER OXIDE AND MIXTURES THEREOF TO OXIDIZE THE ACETYLIDE OF SAID SILVER ACETYLIDE, TO OXIDIZE SAID POLYMER AND TO REDUCE THE SILVER OF SAID SILVER ACETYLIDE TO FREE SILVER WHEREBY SAID CONTAMINANTS ARE EFFECTIVELY CONVERTED TO OXIDATION AND REDUCTION PRODUCTS, AND OXIDIZING SAID FREE SILVER TO SILVER IONS TO REGENERATE SAID SOLUTION. 